Department

Date of Defense

First Committee Member

Second Committee Member

Third Committee Member

Lynne Fieber - Committee Member

Fourth Committee Member

Danielle McDonald - Committee Member

Fifth Committee Member

Kathleen Gilmour - Outside Committee Member

Abstract

Natural exposure to hypersaline environments occurs in many fish species (e.g. in tidal pools, lakes, estuaries, or due to migration). Fish in environments that are more concentrated than their extracellular fluids experience continual diffusive water loss which they combat by drinking. Intestinal Na+ and Cl- absorption drives water uptake and concentrates impermeable MgSO4 in the lumen. Epithelial Cl- absorption occurs in part by apical Cl-/HCO3- exchange, with HCO3- provided by transepithelial transport and/or by carbonic anhydrase-mediated hydration of endogenous CO2. Hydration of CO2 also liberates H+, which is excreted across either the basolateral or apical membranes. Transport of water, ions, and acid-base equivalents are therefore intricately linked in the response of teleosts to high salinity. This dissertation compares three teleost species of varying salinity tolerance: the freshwater, anadromous rainbow trout (Oncorhynchus mykiss), the marine gulf toadfish (Opsanus beta), and the highly euryhaline killifish (Fundulus heteroclitus). In toadfish, intestinal HCO3- secretion increases with salinity, but the resulting acid-base disturbance is compensated by branchial net acid excretion. Toadfish exposed to hypersaline waters containing artificially low [MgSO4] have enhanced survival and osmoregulation compared to fish in water of natural ionic composition. Upper salinity tolerance is determined by intestinal capacity for Na+, Cl-, and water absorption, which is limited by luminal [MgSO4], rather than renal or branchial processes. Rainbow trout exposed to 70% seawater adjust from hyper- to hypoosmoregulatory strategies within 24-48 hours. Ion transporters important to both intestinal osmoregulation and maintenance of acid-base balance (NHE1, NHE2, SLC26a6) in the trout differed from other teleost species in their contribution to intestinal osmoregulation. Methods utilizing isolated killifish intestinal tissue under physiologically relevant conditions were discovered to be ineffectual. Regardless, the extremely salinity-tolerant killifish was shown to require relatively few physiological changes following acute transfer from 35 to 70 ppt seawater, and epithelial permeability adjustments occur within 24 h post-transfer. This dissertation examines the contribution of intestinal transport of water, ions, and acid-base equivalents to the osmoregulation of fish in hypersaline environments. Comparisons among species exposed to their upper salinity tolerance range expand our understanding of the essential iono- and osmoregulatory requirements of teleost fish.